What Guys Said 7

Because the first episode is the pilot. The amount of viewers during the pilot of a tv series indicates how much faith the producers and the other people who invested money into the show should have. And it mostly indicates if the show will fail or not.

A pilot episode is essentially a way of testing the waters before a full series is made. Usually if the pilot episode is a success then it will lead to a series. If not the series is binned and no further episodes are made.